EP-A-0909304 illustrates a process wherein a base oil having a viscosity index (VI) of 95 is prepared from a vacuum distillate boiling between 418 (5 wt % recovery) and 564° C. (95 wt % recovery) by subjecting the feed to a hydrocracking step using a catalyst based on Nickel and Molybdenum. The high boiling part of the hydrocracker effluent was subsequently dewaxed using a ZSM-5 based dewaxing catalyst and hydrofinished using a platinum/palladium based catalyst. The yield to base oil was 62 wt %.
WO-A-0250213 describes a process to prepare a base oil from the high boiling fraction of a fuels hydrocracker process. In this process the high boiling fraction is separated into different distillate fractions which are in turn subjected to a catalytic dewaxing step and a hydrofinishing step.
U.S. Pat. No. 5,525,209 describes a fuels hydrocracker process wherein the bottoms fraction in which bottoms fraction may potentially yield a base oil having a desired high viscosity index value. It is shown in this publication that the viscosity index of the base oil will increase at higher conversion in the hydrocracker step.
According to general textbooks on base oil manufacturing hydrocracking will reduce the viscosity of the feedstock, remove most of the nitrogen, oxygen and sulphur present in the base oil feedstock and convert the undesirable low VI materials such as polynuclear aromatics and polynuclear naphthenes to higher VI materials such as mononuclear aromatics, mononuclear naphthenes and iso-paraffins (Chapter 6 and especially page 122 of Lubricant Base Oil and Wax Processing, Avilino Sequeira, Jr, Marcel Dekker Inc, New York, 1994, ISBN 0-8247-9256-4).
A disadvantage of the above processes is that not all crude derived feedstocks are suitable for preparing a base oil having the desired VI. It may also be possible that a crude derived feed is suitable to meet the VI requirements of some but not all of the desired viscosity grades. This could for example be due to the fact that the content of polynuclear aromatics and naphthenics in the relevant feed or feed fraction are too high. It may sometimes be possible to meet the VI requirements by increasing the hydrocracker conversion as explained above. However such a higher conversion will significantly lower the final base oil yield and may even make it impossible to prepare the heavier grades.
EP-A-921184 describes a process wherein a Fischer-Tropsch wax is added to a crude derived oil. This mixture is used as feed to a hydrocracker. The effluent of the hydrocracker is distilled and a bottom fraction is recovered. This distiller bottom fraction is subjected to a solvent dewaxing treatment to obtain a base oil having a viscosity index of 145 or greater and a kinematic viscosity at 100° C. of between 4.6 and 6.3 cSt.
According to EP-A-921184 the Fischer-Tropsch wax to be used in the disclosed process is isolated from the Fischer-Tropsch synthesis product by only distillation. Typically more than 80% by volume has a boiling point higher than 550° C. One such wax was exemplified and because a substantially normal-paraffinic mixture is expected for such a direct Fischer-Tropsch wax fraction a congealing point of around 100° C. is estimated. This wax was mixed with a petroleum based waxy distillate having a final boiling point of 579° C. and the mixture was subjected to a hydrocracking step. From the examples it can be seen that when the Fischer-Tropsch wax containing feed was used a large fraction boiling above 635° C. was found in the effluent of the hydrocracker.
A disadvantage of the process according to EP-A-921184 is that a large portion of the valuable Fischer-Tropsch molecules added to the hydrocracker feed do not end up in the final base oils.
It would be useful to provide a process to make base oils from a crude derived feedstock wherein use is made of a Fischer-Tropsch derived product in a more efficient manner